The Enteroaggregative Escherichia coli (EAggEc) are a complex group of bacterial organisms which have a distinctive phenotype and a strong association with persistent diarrheal disease in children in the developing world and in patients infected with the Human Immunodeficiency Virus (HIV). To date, however, there are very few hard data about EAggEc that permit speculation about the molecular basis for pathogenesis or development of additional diagnostic methodologies. While very elegant work has been done to describe the genes which are responsible for encoding the AAF/1 fimbriae in some EAggEc strains, it is not possible at present to implicate AAF/1 as the fimbrial adhesin which is present in all or even the majority of EAggEc strains. None of the other work that has been done to date on serogroups of EAggEc strains, entero or cytotoxins, invasive ability, clump formation or hemagglutinins can explain the pathogenesis of diarrhea associated with these organisms or serve as a definitive means of identifying clinically important EAggEc strains as these factors have not been identified in the majority of EAggEc strains from various geographic locations. Not all patients colonized with Enteroaggregative Escherichia coli are symptomatic. This may be due to differences in the host or differences in the colonizing bacteria or a combination of the two. The purpose of this pilot proposal is to address the question of whether the EAggEc function as an opportunistic pathogen, able to cause diarrheal disease in very selected circumstances, such as young or malnourished children in the developing world or patients with immune compromise, such as HIV. This issue will be addressed by undertaking molecular characterization of EAggEc isolates from patients in whom "host factors" which may be related to EAggEc pathogenesis can be measured. A longitudinal clinical study will be performed which will compare patients who are symptomatic when infected with EAggEc and those who are not. Strains from these two groups of patients will be compared, using molecular techniques.